Control of soil-borne plant pests using glucosinolate-containing plants

Isothiocyanates (ITCs) released from Brassica crops or seed meal amendments incorporated into soil have the potential to suppress pest and disease organisms in soil. We investigated in vitro toxicity of six ITCs to the mycelial growth of five cereal root pathogens (Gaeumannomyces graminis var. tritici, Rhizoctonia solani, Fusarium graminearum, Bipolaris sorokiniana, Pythium irregulare) by either adding them to the headspace above, or dissolving them in the growing media. Four alkenyl aliphatic ITCs (methyl-ITC, propenyl-ITC, butenyl-ITC, pentenyl-ITC) and two aromatic ITCs (benzyl-ITC and 2-phenylethyl-ITC) were tested. Aromatic ITCs were less toxic in the headspace experiments due to their lower volatility which reduced the headspace concentration, but were more toxic than the aliphatic ITCs when dissolved in the agar. In both experimental methods, the toxicity of the aliphatic ITCs decreased with increasing length of the side chain although there was little difference between methyl-ITC and propenyl-ITC in the headspace experiment. The fungi differed in sensitivity to the ITCs. Gaeumannomyces was the most sensitive, Rhizoctonia and Fusarium intermediate and Bipolaris and Pythium the least sensitive. Pythium was 2–16 times more resistant than the other fungi to the ITCs dissolved in agar and, in contrast to the other fungi, was more sensitive to the aliphatic ITCs than the aromatic ITCs. Suppression of some fungi by propenyl ITC and 2-phenylethyl ITC, principal products of glucosinolate hydrolysis in Brassica tissue, was superior to that of the synthetic fumigant methyl-ITC, suggesting an important role for these compounds in the pest suppression potential of brassicas. The variation in toxicity of different ITCs to the fungi (up to 7 fold) suggests there is significant scope to enhance the biofumigation potential of brassicas by selecting those which produce large quantities of GSL precursors to the most toxic ITCs for the target organism.

Biofumigation potential of brassicas

Glucosinolates in crop plants

The presence of root tissue of the brassicas canola and Indian mustard inhibited growth of pure cultures of the fungal pathogen which causes take-all of wheat [Gaeumannomyces graminis (Sacc.) Arx and Oliver var.tritici, abbreviated as Ggt]. Ggt growth was generally inhibited more in the presence of Indian mustard roots than canola roots. Dried irradiated roots were consistently effective in reducing Ggt growth, but growth inhibition by young live roots and macerated roots was not consistent. The inhibitory compound(s) were shown to be volatile because the symmetry of Ggt growth was not affected by the proximity of theBrassica tissue. Volatile breakdown products from maceratedBrassica roots were identified using a gas chromatograph-mass spectrometer. The major compounds found were isothiocyanates (ITCs). Canola roots released mostly methyl ITC and Indian mustard roots released mostly phenylethyl ITC. Low concentrations of these and related compounds inhibited growth of Ggt in pure culture when supplied as the vapour of pure chemicals in concentrations within the range expected during breakdown ofBrassica roots in soil.

Biofumigation: Isothiocyanates released from Brassica roots inhibit growth of the take-all fungus

Biofumigation refers to the suppression of soil-borne pests and pathogens by biocidal compounds released in soil when glucosinolates (GSL) in Brassica green manure or rotation crops are hydrolysed. We investigated the potential to enhance biofumigation by considering the variation in GSL production in the roots and shoots of 76 entries from 13 Brassica and related weed species grown in the field. Total plant GSL production on a ground area basis at mid-flowering ranged from 0.8 to 45.3 mmol m -2 . The variation derived equally from differences in biomass and GSL concentration, which were not correlated in either root or shoot tissues. Roots (0-0.15 m) contributed an average of 23.6% (range 2-81%) of the total plant GSLs, their contribution limited by low biomass rather than GSL concentration, which was usually similar or higher than that of shoots. The GSL concentrations in root and shoot tissues did not correlate significantly with seed levels in any of the species, so selection for higher plant GSL production to enhance biofumigation potential cannot be based on seed GSL levels. The types of GSLs present in the tissues varied considerably between species but were consistent within species. In contrast, the concentration of individual and total GSLs in both root and shoot tissues varied four to ten-fold both between and within all species. Shoots contained predominately aliphatic GSLs, while aromatic GSLs, particularly 2-phenylethyl GSL, were dominant in the roots of all entries. Indolyl GSLs were present in all tissues but at low concentrations (< 1 μmol g -1 ). The variation in the biomass, GSL profiles and concentrations in both roots and shoots provide significant scope to select or develop brassicas with enhanced biofumigation potential. Further studies on the efficacy of the various GSL hydrolysis products to suppress target organisms in soil are required to fully exploit biofumigation as a part of integrated pest management.

Biofumigation potential of brassicas. I. Variation in glucosinolate profiles of diverse field-grown brassicas

A number of glucosinolates and isothiocyanates were tested for their allelopathic potential against wheat. Most of the glucosinolates showed no activity against wheat, with the exception of glucobrassicin which was moderately active, as was sinapine thiocyanate. Isothiocyanates showed high activity against wheat germination and seedling growth. The most active compound, 2-phenethyl ITC completely inhibited wheat germination at 500 ppm. Allyl ITC showed high activity whereas other isothiocyanates tested were only moderately active. The data is discussed in relation to the possible use of some mustard species for effective weed control.

J. Lewis

Papers

Allelopathic potential of glucosinolates (mustard oil glycosides) and their degradation products against wheat